There are a number of applications in which a small number of bits of non-volatile memory are needed in an environment in which power is not always available either to program the memory or to maintain the memory. Consider a memory that monitors a line and records the occurrence of an event on the line if the event occurs. Power is only available during the time the event takes place. That is, the only power is the power in the event signal. In addition, the power in the signal may be quite small. In principle, the memory can be provided with its own power source such as a battery to power the memory and maintain the circuitry in a monitoring state. However, such arrangements require the batteries to be changed and significantly increase the cost of the memory.
Ideally, the memory should be able to monitor the line over an extended period of time and record an event on the line without using an independent power source. When power is applied to the memory at some subsequent time, the memory should assume a state that reflects whether or not an event has taken place during the period of time that the memory was not powered.